Color improvement of phosphate esters

ABSTRACT

A method for decolorizing and stabilizing alkylphenyl esters of phosphoric acid which comprises contacting the esters with an effective amount of a sodium salt selected from the group consisting of sodium dithionite and sodium formaldehyde sulfoxylate for a sufficient length of time to decolorize and stabilize the esters against subsequent color formation.

BACKGROUND OF THE INVENTION

This invention relates to the decolorization and stabilization ofalkylphenyl esters of phosphoric acid. The production of low color andcolor stable phosphate esters from alkylphenol precursors has been along recognized problem. In the conventional production of phosphateesters from alkylphenols, undesirable color formation frequently occurs.Color can decrease the phosphate ester's value as a commercial product.

Alkylphenyl esters of phosphoric acid find extensive use as plasticizersfor nitrocellulose and polyvinyl chloride (PVC) compositions. Inaddition, they also serve as additives for gasoline, functional fluids,oils, and are useful as flame retardants in plastics, and the like.

The preparation of alkylphenyl esters of phosphoric acid is generallyaccomplished by the addition of phosphorus oxychloride, (POCL₃) toselected phenols, such as cresols, xylenols, and the like, and graduallyheating the resulting reaction mixture to about 180°C. The reaction isaccelerated by the presence of a Friedel-Crafts catalyst such asaluminum chloride, (AlCl₃). In conventional processing, the reactionproduct is vacuum distilled to remove unreacted phenols as an initialfraction, and the alkylphenyl phosphate ester as a product fraction,leaving high boiling point materials and the catalyst in the residue.The distilled product fraction is then washed thoroughly with sodiumhydroxide solution to remove free phenol and acidic materials, followedby water washing. The product fraction is then generally treated withactivated carbon to remove color causing impurities.

This process, which works very well with esters produced fromconventional by-product alkylphenols, produces unsatisfactory materialwhen applied to mixed alkylphenols produced by the alkylation of phenolwith olefins. It is found that the products discolor upon exposure toair, exposure to heat, or storage in the dark. The discoloration hasbeen attributed to the presence of di(o-alkyl) phenols in the alkylatedphenol. Steric hindrance caused by the double ortho substitution in2,6-dialkylphenols and in 2,4,6-trialkylphenols renders the phenolsunresponsive to washing with sodium hydroxide solution, so that they arenot removed by the caustic wash.

These so-called "hindered phenols," can oxidize in the presence of airto form highly colored quinones, which are the source of undesirablediscoloration in the product. These quinones can bleach somewhat in thelight, however, color will reappear upon storage in the dark. The colorcan intensify when the ester is mixed or milled with polyvinyl chloride(PVC) under the influence of air and heat.

Thus, for example, in the case of 2,6-diisopropylphenol, thecorresponding diphenoquinone or benzoquinone is highly colored. Estersmade from mixtures of alkylated phenols which containdi-orthoalkylphenols such as 2,6-diisopropylphenol can be too highlycolored for many uses, particularly for use as plasticizers. High colorphosphate esters have limited utility for plasticizer use and are lessmarketable.

Color formation in phosphate esters is discussed in U.S. Pat. No.3,681,482, which correlates the degree of color formation in phosphateesters to the degree of alkyl substitution of the aryl ring.Accordingly, tris(methylphenyl) phosphate will produce less color thantris(dimethylphenyl) phosphate. This may be explained by the fact thatthe tris(methylphenyl) phosphate has only one methyl group substitutedon the aryl ring, whereas tris(dimethylphenyl) phosphate has two methylgroups substituted on the aryl ring and, therefore, has more color.

A number of proposals have been made for methods to overcome the problemof undesirable color formation. For example, U.S. Pat. No. 1,958,210discloses the use of activated carbon to decolorize and removeoxidizable impurities from phosphate esters. This approach isunsatisfactory because activated carbon is not an effective decolorizingagent for alkylphenyl phosphate esters. In certain instances, forexample, in the decolorization of isopropylphenyl diphenyl phosphateester, the use of activated carbon may increase color formation.

U.S. Pat. No. 2,113,951 discloses a method wherein an alkylphenol suchas cresylic acid is distilled in the presence of a mineral acid such assulfuric, hydrochloric or phosphoric acid, to purify it. The purifiedcresylic acid is then employed in the manufacture of tricresyl phosphateesters which are supposed to be more stable to the action of heat andlight than the corresponding ester made from alkylphenols distilled inthe absence of an inorganic acid. The disadvantage of this process isthat the phenolic residues oxidize to colored quinones, and must bethoroughly distilled in order to remove them and avoid further colorformation.

Another method for reducing color is proposed in U.S. Pat. No. 3,681,482wherein sodium borohydride is used to permanently bleach and colorstabilize tris(alkylphenyl)phosphate esters containing2,6-diisopropylphenol and the corresponding diphenoquinone. The sodiumborohydride reduces the diphenoquinone to the colorless2,6-diisopropylphenol which, however, remains in the product and is apotential source of discoloration if the product is exposed to oxidizingconditions. Sodium borohydride treatment is also expensive in cost ofmaterials and time, as several hours to overnight treating times arenecessary.

Thus, it can be seen that the methods proposed in the prior art, are notcommercially effective for removing color from alkylphenol esters ofphosphoric acid, or do not improve the PVC mill stability when theseesters are used as PVC plasticizers.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for removing colorfrom alkylphenyl esters of phosphoric acid which also improves the millstability of these esters when they are used as plasticizers for PVC hasnow been discovered.

The method comprises contacting the alkylphenol phosphate esters with aneffective amount of a sodium salt selected from the group consisting ofsodium dithionite and sodium formaldehyde sulfoxylate. The phosphateester is decolorized and stabilized against subsequent formation. Thistreatment produces a low color, stable phosphate ester and can beconducted during the sodium hydroxide wash of the phosphate esterproduct, or as a separate treatment after the sodium hydroxide washingstep. The preferred alkali metal salts encompassed in this inventioninclude sodium dithionite (Na₂ S₂ O₄) and sodium formaldehydesulfoxylate (HOCH₂ SO₃ Na). The equivalent potassium salts alsofunction. These salts are generally employed in an aqueous solution.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

Permanent removal of color from colored phosphate esters according tothe method of this invention is achieved by contacting the phosphateester with an effective amount of a sodium salt selected from the groupconsisting of sodium dithionite and sodium formaldehyde sulfoxylate inaqueous solutions for a time sufficient to decolorize the phosphateester to the desired level, and stabilize it against subsequent colorformation. This contacting is accomplished during the caustic washing orsubsequent water washing of the phosphate ester.

This invention is applicable to all phosphate esters which are made fromalkylated phenol mixtures which contain hindered phenols, e.g., phenolscontaining alkyl groups on both positions ortho to the hydroxyl group.The esters may contain 0.5 to 3 alkylaryl groups and 0 to 2.5 phenylgroups. Preferably, the triaryl phosphate esters treated by the processof this invention are a mixture of esters containing 1 to 2 alkarylgroups. The esters correspond to the general formula: ##EQU1## where Ris alkaryl and R₁ and R₂ may be alkyl, alkaryl, aralkyl or aryl, andwherein the alkyl groups can contain from 1 to 20 carbon atoms and morepreferably, from 1 to 12 carbon atoms. Some triphenylphosphate may alsobe present.

The alkylated phenols which contain hindered phenols are usually made byalkylating phenol with C₂ -C₁₂ unsaturated hydrocarbons such asethylene, propylene, isobutylene and its isomers, amylene and itsisomers, tripropylene, tetrapropylene, decene, dodecene, diisobutyleneand the like.

Typical examples of alkyl radicals are as follows:

methyl, ethyl, normal propyl, isopropyl, normal butyl, isobutyl,secondary butyl, tertiary butyl, normal amyl, isoamyl, 2-methylbutyl,2,2-dimethyl propyl, 1-methyl butyl, diethylmethyl, 1,2-dimethyl propyl,tertiary amyl, normal hexyl, 1-methylamyl, 1-ethyl butyl,1,2,2-trimethyl propyl, 3,3-dimethyl butyl, 1,1,2-trimethyl propyl,2-methyl amyl, 1,1-dimethyl butyl, 1-ethyl 2-methyl propyl, 1,3-dimethylbutyl, isohexyl, 3-methylamyl, 1,2-dimethyl butyl, 1-methyl 1-ethylpropyl, 2-ethyl normal heptyl, 1,1,2,3-tetramethyl propyl, 1,2-dimethyl1-ethyl propyl, 1,1,2-trimethyl butyl, 1-isopropyl 2-methyl propyl,1-methyl 2-ethyl butyl, 1,1-diethyl propyl, 2-methyl hexyl, 1,1-dimethylamyl, 1-isopropyl butyl, 1-ethyl 3-methyl butyl, 1,4-dimethyl amyl,isoheptyl, 1-methyl 1-ethyl butyl, 1-ethyl 2-methyl butyl, 1-methylhexyl, 1-propyl butyl, normal octyl, 1-methyl heptyl, 1,1-diethyl2-methyl propyl, 1,1,3,3-tetramethyl butyl, 1,1-diethyl butyl,1,1-dimethyl hexyl, 1 -methyl 1-ethyl amyl, 1-methyl 1-propyl butyl,2-ethyl hexyl, 6-methyl heptyl normal nonyl, 1-methyl octyl, 1-ethylheptyl, 1,1-dimethyl heptyl, 1-ethyl 1-propyl butyl, 1,1-diethyl3-methyl butyl, diisobutyl methyl, 3,5,5-trimethyl hexyl, 3,5-dimethylheptyl, normal decyl, 1-propyl heptyl, 1,1-diethyl hexyl, 1,1-dipropylbutyl, 2-isopropyl 5-methyl hexyl and C₁₁ -C₂₀ alkyl groups.

Also included are aralkyl groups, e.g., benzyl, alpha- or beta-phenylethyl, alpha,alpha dimethyl benzyl and the like. Also included arecyclohexyl, cycloheptyl, cyclododecyl and the like.

Typical examples of aryl and alkaryl radicals are phenyl, cresyl, xylyl,alkoxylated phenyl, isopropylphenyl, butylphenyl,alpha-alkylbenzylphenyl and alpha,alpha-dialkylbenzylphenyl, e.g.,alpha-methylbenzylphenyl, alpha,alpha dimethylbenzylphenyl,tert-nonylphenyl, amylphenyl, tert-butylphenyl, isooctylphenyl,dodecylphenyl, tertiary octylphenyl and the like.

The invention is hereinafter exemplified by first showing thepreparation of an ester via the alkylation of phenol with an olefin,followed by addition of POCl₃. These alkylated phenols produce esterswhich are similar to those produced with conventional by-product coaltar cresylic acids or methylphenols.

The esters are generally made by reacting an alkylphenol with POCl₃ inthe presence of a Friedel-Crafts catalyst at an elevated temperature,typically about 180°C., until the reaction is complete, as noted by thecessation of HCl evolution. The reaction mixture is then heated todistill excess phenols overhead. The temperature and/or vacuum is thenincreased and the phosphate ester product is distilled leaving thecatalyst and a small amount of high boiling distillation residue.

Conventionally, the ester product is washed with aqueous alkali toremove free phenols which are generally present in the range of about afew tenths of a percent. The washed product is separated from the waterand generally treated with activated carbon and a filter aid, such asdiatomaceous earth, and filtered. However, product discoloration causedby hindered phenols in the presence of air and heat can ensue, renderingthe product unsuitable for use in applications where lack of color isimportant.

In accordance with this invention, the phosphate ester is treated forcolor removal and color stabilization by contacting the alkylphenylphosphate ester with an effective amount of an alkali metal salt of areduced form of sulfur until the color is adequately reduced. Thecontacting treatment may be conducted during the sodium hydroxide washof the phosphate ester product, or as a separate treatment of thefinished product. The sodium salts encompassed by the process of thepresent invention include sodium dithionite and sodium formaldehydesulfoxylate and mixtures thereof. The equivalent soluble salts of othercations also function. Included are potassium, lithium, ammonium,calcium, and mixtures thereof.

Treatment times will vary, generally from about 5 minutes to about 24hours, depending upon the amount of phosphate ester treated, the amountand concentration of the sodium salt in solution, the temperature,agitation, and the like. Preferably, the phosphate esters are washed forabout 10 minutes to about 5 hours at a temperature of about 20°C. toabout 100°C. More preferably, the washing treatment is carried out attemperatures of about 25°C. to about 70°C. for about 20 to about 120minutes.

The amount of water soluble salt in solution can vary in amount fromabout 0.1% to about 10% by weight of the phosphate ester treated. Largeramounts of the water soluble salt can be employed, but no advantage isaccrued thereby. It is preferred to use an amount ranging from about0.5% to about 5% by weight of the phosphate ester with about 1% to about3% being particularly preferred. The particular amount of water solublesalt employed in any given instance will to some extent be influenced bya number of factors which include the amount of color present, theextent of color improvement desired, the particular phosphate estertreated, treatment time, and the like.

The method of this invention is generally conducted under atmosphericpressure. However, higher or lower pressures may be used. It may also beconducted under an inert atmosphere, such as nitrogen which serves torepress re-oxidation.

The water soluble salts of this invention are preferably employed inaqueous solutions. The method of this invention may be carried outbatch-wise or in a continuous manner.

One particular advantage of the instant invention is that aftertreatment of the phosphate ester with the soluble salt solution, noadditional steps or special treatment other than an optional waterwashing step, phase separation and drying are necessary. Trace residualwater soluble salts of a reduced form of sulfur if present, appear notto have an adverse effect on the commercial properties of the phosphateesters depending upon the particular salt involved. However, an optionalfinal water wash can be employed primarily to remove soluble inorganicmaterials prior to drying.

The following examples are illustrative of the methods disclosed above,and are provided without any intention that the invention be limitedthereto. In the examples and throughout the specificaton, all parts andpercentages are by weight, unless otherwise noted.

EXAMPLE 1 Preparation of Isopropylphenyl Diphenyl Phosphate Ester

1.86 Moles of phenol, 1.50 moles of an isopropylphenol mixturecontaining mono- and diisopropyl phenols, and 1.12 moles of phosphorusoxychloride were placed in a 1 liter reactor and stirred at a moderaterate at room temperature. Nitrogen gas was bubbled slowly through theliquid and anhydrous magnesium chloride catalyst was added to thestirred contents. The charge was then heated, while slowly beingstirred, from a temperature of 38°C. to 180°C. over a period of fourhours. The temperature was maintained at 180°C. for approximately fivehours or until the evolution of hydrogen chloride ceased.

The crude isopropylphenyl diphenyl phosphate product weighing 423 gramswas then transferred to a 1/2 liter distillation flask and vacuumdistilled through a 6 × 1 inches column packed with 1/4 inch glassspheres. The distilled isopropylphenyl diphenyl phosphate ester productweighing 366 grams was then introduced into a 2 liter reactor and washedfor one hour at 65°C. with 300 milliliters of a 1% sodium hydroxidesolution. The caustic wash was repeated two additional times with freshcaustic solution.

The isopropylphenyl diphenyl phosphate ester was then separated from thecaustic solution and washed twice with 300 milliliters of water at 65°C.for one hour. The water phase after the second wash was neutral to pHpaper. The washed ester is separated from the final wash water and driedat 100°C. under a vacuum of 1.0 millimeters Hg (5-10 millimeters Hg at100°C. is also satisfactory), until no more water distills over. Thedried ester is filtered to give 351 grams of finished ester product.

EXAMPLE 2 Treatment of Distilled Unwashed Isopropylphenyl DiphenylPhosphate Ester

200 gram samples of an isopropylphenyl diphenyl phosphate ester having aSaybolt Universal Viscosity of 220 seconds prepared in a manner similarto that of Example 1 and having an APHA color value of 75 (AmericanPublic Health Associaton Platinum-Cobalt Scale for designating color)were each treated for 5 hrs. at 65°C. with 200 ml. of a 1% NaOH solutioncontaining two grams of one of the reagents listed in the table below.The caustic solution was separated and the ester was washed two timeswith an equal volume of water. The residual water was removed by vacuumstripping at 80°C/2.0 mm. to give an ester with an improved color. Thecolors achieved are given in the table below for each reagent tried.

                         APHA Color Treated                                       Reagent              Ester                                                    ______________________________________                                        Sodium dithionite    25-50                                                    Sodium formaldehyde sulfoxylate                                                                    25-50                                                    Control (no reagent)  75                                                      Caustic, Air (no reagent added)                                                                    150                                                      ______________________________________                                    

The sodium salt treated ester samples were tested for PVC mill stability(which determines the effect of mill conditions on the darkeningproperties of a vinyl plastic film formulation incorporating thephosphate ester as a plasticizer). Where the sodium salt ester wasuntreated, but was washed only with dilute sodium hydroxide and thenwater, the phosphate ester failed the PVC mill stability test.

Storage tests were also performed on the sodium salt treated esters. Aportion of each ester contacted with a sodium salt of a reduced form ofsulfur was placed in a dark closet for 10 days with no change in color.

EXAMPLE 3 Post Treatment of Another Finished Isopropylphenyl DiphenylPhosphate Ester

Two 200 gram samples of a finished isopropylphenyl diphenyl phosphateester prepared in a manner similar to that of Example 1 and having aSaybolt Universal Viscosity of 220 seconds and an APHA color value of150, were treated with 3 grams of each reagent listed in the tablebelow, dissolved in 40 ml. water at 65°C. for a 5 hour period. Eachester was then washed two times with an equal volume of water andstripped of residual moisture at 80°C/2.0 mm. vacuum to give a treatedester having a lower APHA color value.

    ______________________________________                                                            APHA Color                                                Reagent             Treated Ester                                             ______________________________________                                        Na.sub.2 S.sub.2 O.sub.4                                                                          50                                                        NaHSO.sub.3 .sup.. CH.sub.2 O .sup.. 2H.sub.2 O                                                   50                                                        ______________________________________                                    

The treated samples passed the test for PVC mill stability and whenplaced in a dark closet for 10 days exhibited no change in color.

EXAMPLE 4 Illustrative Embodiment

When separate samples of isopropylphenyl phenyl phosphate are treatedwith 1% by weight of sodium dithionite and sodium formaldehydesulfoxylate in accordance with the procedure of Example 2, improved APHAcolor values will result. In addition, the treated samples will pass thetest for PVC mill stability, and when placed in a dark closet for 10days will not change color.

What is claimed is:
 1. A method for producing alkylphenyl esters ofphosphoric acid which comprises the alkylation of phenol with an olefinfollowed by the addition of POCl₃ in the presence of a Friedel-Craftscatalyst at an elevated temperature the improvement which comprisescontacting said esters with an effective amount of a water-soluble saltof a dithionite or formaldehyde sulfoxylate selected from the groupconsisting of sodium, potassium, lithium, ammonium, calcium, andmixtures thereof, for a period of time sufficient to reduce the color tothe desired level whereby said esters are decolorized and stabilizedagainst subsequent color formation.
 2. The method of claim 1 whereinsaid alkylphenyl esters correspond to the formula: ##EQU2## wherein R isalkaryl and R₁ and R₂ are selected from the group consisting of alkyl,aralkyl, alkaryl and aryl, and wherein the alkyl groups contain from 1to 20 carbon atoms.
 3. The method of claim 2 wherein said alkyl groupscontain from 1 to 12 carbon atoms.
 4. The method of claim 1 wherein thealkylphenyl esters contain unreacted phenols.
 5. The method of claim 1wherein said water soluble salt is selected from the group consisting ofsodium, potassium and mixtures thereof.
 6. The method of claim 5 whereinsaid water soluble salt is selected from the group consisting of sodiumdithionite, sodium formaldehyde sulfoxylate, and mixtures thereof. 7.The method of claim 5 wherein said water soluble salt is selected fromthe group consisting of potassium, dithionite, potassium formaldehydesulfoxylate, and mixtures thereof.
 8. The method of claim 5 wherein saidalkali metal salt is employed in an aqueous solution.
 9. The method ofclaim 1 wherein the alkyl phenyl ester of phosphoric acid is anisopropylphenyl phenyl phosphate.
 10. The method of claim 1 wherein saiddecolorizing and stabilizing is conducted under an inert atmosphere. 11.The method of claim 10 wherein said inert atmosphere comprises nitrogen.12. The method of claim 1 wherein said decolorizing and stabilizing isaccomplished in a dilute sodium hydroxide solution.